Method of manufacturing a tank

ABSTRACT

The invention relates to a method of manufacturing at least one hollow element of a fuel system ( 3 ), especially a casing of a fuel tank. It comprises the following steps: (a) introducing the element or elements to be treated into a treatment chamber ( 2 ) equipped with at least one microwave generator, (b) injecting a reaction gas into the chamber while enabling the gas to enter inside the said elements, (c) subjecting this gas to microwave radiation delivered by the microwave generator ( 4 ), at a frequency and intensity chosen so as to create a plasma in the chamber and to bring about the coating of a material, arising from the decomposition of the reaction gas, on the said element or elements, preferably on both the inner and outer faces of the said element or elements.

[0001] The present invention relates to a method for depositing acoating on an element of a motor vehicle fuel system, especially a tankcasing.

[0002] It is known to treat motor vehicle fuel tanks made of plastic soas to make them impermeable to hydrocarbons.

[0003] Such a treatment may consist in superimposing a layer forming ahydrocarbon barrier onto the plastic constituting the casing of thetank.

[0004] U.S. Pat. No. 5,677,010 describes a method for producing such alayer on the inner surface of a fuel tank, in which a reaction gas isintroduced inside the tank by means of a probe and this gas is activatedusing microwave radiation in order to generate a plasma therein. A layerforming a barrier, arising from the decomposition of the reaction gas,is thus formed on the inner surface of the tank. Several tanks can betreated simultaneously by this method by introducing an individual probeinto each of them. It is thus necessary to provide as many individualprobes as there are tanks to be treated simultaneously, which makes themethod relatively complex to implement.

[0005] German Patent Application DE 44 37 050 describes a device fortreating the inner surface of a fuel tank by means of a plasma. The tankto be treated is placed in a cavity defined by two sheet-metal elements,this cavity being shaped so as more or less to match the shape of thetank.

[0006] French Patent Application FR 2 776 540 describes a method forproducing a layer forming a barrier on the inner or outer surface of abottle, a plasma being generated, depending on the case, in the bottleitself or in the reactor, outside the bottle, the latter then beingstoppered.

[0007] There is a need to improve the performance of motor vehicle fueltanks, in particular their impermeability to constituent components ofthe fuel, in particular the hydrocarbons.

[0008] There is also a need to be able to treat several tankssimultaneously in a relatively simple way.

[0009] The invention meets these needs at least partially using a newmethod of manufacturing at least one hollow element of a fuel system,especially a casing of a fuel tank, characterized in that it comprisesthe following steps:

[0010] (a) introducing the element or elements to be treated into atreatment chamber equipped with at least one microwave generator,

[0011] (b) injecting a reaction gas into the chamber while enabling thegas to enter inside the said elements,

[0012] (c) subjecting this gas to microwave radiation delivered by themicrowave generator, at a frequency and intensity chosen so as to createa plasma in the chamber and to bring about the coating of a material,arising from the decomposition of the reaction gas, on the said elementor elements, preferably on both the inner and outer faces of the saidelement or elements.

[0013] By virtue of the invention, it is possible to produce a tank, thecasing of which is made of plastic and has, on its inner and outersurfaces, a coating layer of a material different from the casing,arising from the decomposition of the reaction gas, chosen to improvethe properties of the tank, for example with respect to hydrocarbons,ultraviolet radiation, abrasion or heat.

[0014] By depositing the material arising from the decomposition of thereaction gas both on the inner and outer faces of the element orelements treated, a double barrier is obtained, which is more effectivethan a single barrier which would be present only on one of the faces ofthe element or elements.

[0015] Furthermore, where the element or elements placed in the chambercomprise(s) one or more attached elements such as a valve, a gaugemounting or an electrical connector in addition to the tank casing, theinner and outer coating layers are deposited both on the casing and onthe attached elements and are not interrupted at the joins between theattached elements and the casing.

[0016] It is thus possible to obtain good impermeability of the assemblyin particular to hydrocarbons, and to comply with the most stringentstandards concerning emissions.

[0017] Preferably, several elements are treated simultaneously in thechamber.

[0018] In this case, advantageously only a single plasma is generatedfor the entire chamber, which avoids having to introduce a multitude ofindividual probes into each of the elements to be treated.

[0019] Advantageously, between the abovementioned steps a) and b), thechamber is evacuated such that the absolute pressure therein is lessthan 0.05 mbar (5 Pa), preferably less than 0.01 mbar (1 Pa).

[0020] It is possible to eliminate the traces of gas which are harmfulto the plasma treatment, in particular oxygen, in a particularlyeffective way by creating a fluorine-based plasma in the chamber afterhaving evacuated it, the oxygen atoms that combine with the fluorineatoms then being pumped out of the chamber.

[0021] The reaction gas may comprise a hydrocarbon compound, preferablywith at least 5 carbon atoms.

[0022] In particular, it is possible to use an alkane or an alkene or amixture thereof.

[0023] Preferably, a mixture of methane and propane is used, such amixture being inexpensive and enabling a layer of carbon with a polymertendency to be deposited onto the tank, constituting an effectivehydrocarbon barrier.

[0024] Such a layer of carbon with a polymer tendency has strongchemical bonds with the substrate and a certain deformability, whichenables it to follow the deformations of the casing to some extent,without a significant risk of failure or of detachment.

[0025] The reaction gas is a siloxane, preferably hexamethyldisiloxanein vapour form.

[0026] The reaction gas can be injected at a nominal pressure of between0.05 mbar (5 Pa) and 0.5 mbar (50 Pa), for example, preferably equal toabout 0.1 mbar (10 Pa).

[0027] The microwaves may be produced at a frequency of between 2 GHzand 3 GHz for example, preferably about 2.45 GHz.

[0028] Advantageously, the microwaves are generated at a power whichvaries with time, so as to reduce the heating of the casings of thetanks and enable the latter to keep their shape throughout thetreatment.

[0029] Advantageously, the aforementioned steps b) and c) are repeatedat least once with a reaction gas which is different from the previousgas(es) in order to deposit several materials onto the element orelements to be treated.

[0030] It is thus possible to produce a tank having at least one layerforming a hydrocarbon barrier but also a layer offering betterultraviolet or abrasion resistance, for example, depending on thematerials deposited on the casing.

[0031] Advantageously, pressure variations are created in the chamber,especially pressure oscillations, so as to make the casing(s) breatheand to improve the diffusion of the plasma thereinside.

[0032] The relative amplitude of these pressure oscillations is, forexample, greater than a factor of five and may be equal to a factor ofabout ten.

[0033] A favoured manner of varying the pressure in the chamberconsists, on the one hand, in applying continuous suction to the chamberand, on the other hand, in injecting the reaction gas with a flow ratewhich varies over time, this injection preferably being carried out byintermittently opening a valve for controlling the supply of thereaction gas.

[0034] The oscillation period may be of the order of a few seconds toseveral tens of seconds.

[0035] The plasma is generated in one particular embodiment of theinvention at ambient temperature, which minimizes the heating and thedeformation of the tank casing.

[0036] Another subject of the invention is an element of a fuel system,especially a tank casing, comprising a substrate layer having an innersurface and an outer surface, characterized in that each of thesesurfaces is coated with at least one coating material which is differentfrom that of the substrate, arising from the decomposition of a chemicalcompound under the effect of plasma-generating microwave radiation.

[0037] Such a coating material may be chosen so as to form a hydrocarbonbarriers or to protect from ultraviolet radiation, to increase theabrasion resistance of the tank or its resistance to heat.

[0038] Advantageously, the coating material comprises carbon with apolymer tendency and/or silica.

[0039] The substrate layer may receive a layer of the same material orseveral layers of different materials on each face, the thickness ofeach layer preferably being between 0.3 and 1 μm.

[0040] The element or elements treated are advantageously made of athermoplastic, especially HDPE.

[0041] As indicated above, the coating material deposited on thesubstrate layer is preferably bonded thereto by strong chemical bonds.

[0042] In the case of carbon, these strong chemical bonds are obtainedby the surface carbon atoms of the substrate layer combining with theatoms of the coating material.

[0043] As indicated above, the fuel system element treated according tothe invention may comprise, other than a casing, attached elements suchas a valve or an electrical connector, assembled before the plasmatreatment.

[0044] The subject of the invention is also a fuel tank comprising, onat least one of its inner or outer faces, a coating of carbon or ofsilica with a polymer tendency.

[0045] Other characteristics and advantages of the present inventionwill emerge on reading the following detailed description of anon-limiting exemplary embodiment of the invention, and on examining thesingle FIGURE showing very schematically a plant for treating fueltanks.

[0046] A treatment plant 1 according to the invention is shown in thedrawing, comprising a treatment chamber 2 in which the tanks 3 to betreated are placed.

[0047] These tanks 3 have been produced, in the example in question, byblow moulding high-density polyethylene.

[0048] Of course, the invention is not limited to the treatment of tanksmade by blow moulding and is in general applicable to the treatment oftanks obtained by thermoforming, rotomoulding or injection moulding, forexample.

[0049] Each tank 3 is especially equipped with a valve 15 fastenedthereon before its introduction into the treatment chamber 2.

[0050] Each tank may of course be equipped with other attached elements,such as nozzles, gauge mountings or electrical connectors, in aconventional manner.

[0051] The chamber 2 is equipped with a magnetron 4, known per se,enabling a plasma, to be created thereinside, as will be explainedbelow.

[0052] The nominal power of the magnetron 4 is pulsed at a lowfrequency, it being possible for the duration of the pulses to vary from0.5 to 30 s, so as to avoid excessive heating of the tanks 3, whichcould cause their deformation.

[0053] The tanks 3 are held in the chamber 2 by support means which havenot been shown for the sake of clarity in the drawing.

[0054] The plant 1 comprises a two-stage suction system 5 and 6 whichoperates continuously and enables a relatively high vacuum to be createdin the chamber 2, the absolute pressure therein being between 0.01 and0.1 mbar in the example described.

[0055] The chamber 2 is supplied with a reaction gas contained in astorage tank 7 via a solenoid valve 8 connected to a control device 9.

[0056] The chamber 2 may be big enough so that a large number of tankscan be treated simultaneously in batches, it being possible for thisnumber to be greater than 100, for example.

[0057] The plant 1 operates as follows.

[0058] The tanks 3 are introduced into the chamber 2, then the vacuum iscreated.

[0059] Next, a fluorine-based plasma is generated and the residualoxygen atoms combined with the fluorine atoms are eliminated by thetwo-stage suction system 5 and 6.

[0060] Then, a reaction gas contained in the storage tank 7 isintroduced into the chamber by opening the solenoid valve 8.

[0061] In the example in question, this reaction gas consists of amixture of methane and propane and its nominal pressure of injectioninto the chamber 2 is 0.1 mbar.

[0062] The reaction gas introduced into the chamber 2 is ionized by themicrowaves generated by the magnetron 4 while the material constitutingthe casing of the tanks 3 is activated by the same microwaves.

[0063] The use of a reaction gas consisting of a propane and methanemixture makes it possible to deposit amorphous carbon with a polymertendency, bonded by very strong chemical bonds to the materialconstituting the casing of the tanks, onto the inner and outer surfacesof the tanks 3.

[0064] The carbon layer thus deposited is therefore intimately bonded tothe casing of each tank.

[0065] Furthermore, the aforementioned chemical bonds enable the carbonlayer to follow any deformation of the casing while preserving itscontinuity.

[0066] The plasma may be generated at ambient temperature, whichminimizes the heating and the deformation of the tank casings during thetreatment.

[0067] Once the treatment by the reaction gas is completed, a secondreaction gas different from the first can be introduced into the chamber2 in order to deposit a layer of a material giving additional propertiesto the tanks, onto the tanks 3.

[0068] It is thus possible for several materials respectivelyimpermeable to the various compounds constituting petrol, to bedeposited in succession.

[0069] By way of example, a second reaction gas consisting ofhexamethyldisiloxane may be introduced into the chamber 2, which gasenables a silica coating to be obtained on the tanks 3 under the effectof the microwaves.

[0070] The solenoid valve 8 is advantageously intermittently opened andclosed by the control device 9, so as to create pressure oscillations inthe chamber 2, the stages 5 and 6 operating continuously.

[0071] Variations of a factor 10 of the pressure inside the chamber 2can thus be produced.

[0072] These pressure variations enhance the penetration of the reactiongas inside the tanks.

[0073] Of course, the invention is not limited to the exemplaryembodiments which have just been described.

[0074] In particular, it is possible to introduce more than two reactiongases into the treatment chamber 2 in succession, so as to depositvarious materials chosen according to the properties that it is desiredto give the tanks.

[0075] In particular, it is possible to deposit various materials aimingto solve problems of conductivity, ultraviolet resistance, abrasionresistance, resistance to given chemicals or sensitivity to heat orflame.

[0076] The tanks 3 may be treated while they are without an attachedelement.

[0077] The invention is not limited to treating fuel tanks for motorvehicles, but is applicable more generally to any type of tank.

1. Method of manufacturing at least one hollow element of a fuel system(3), especially a casing of a fuel tank made of plastic, characterizedin that it comprises the following steps: (a) introducing the element orelements to be treated into a treatment chamber (2) equipped with atleast one microwave generator, (b) injecting a reaction gas into thechamber while enabling the gas to enter inside the said elements, (c)subjecting this gas to microwave radiation delivered by the microwavegenerator (4), at a frequency and intensity chosen so as to create aplasma in the chamber and to bring about the coating of a material,arising from the decomposition of the reaction gas, on the said elementor elements, preferably on both the inner and outer faces of the saidelement or elements.
 2. Method according to the preceding claim,characterized in that, between the steps a) and b), the chamber isevacuated such that the absolute pressure therein is less than 0.05 mbar(5 Pa).
 3. Method according to any one of the preceding claims,characterized in that the reaction gas is a hydrocarbon compound,preferably with at least 5 carbon atoms, in particular an alkane or analkene or a mixture thereof, preferably a mixture of methane andpropane.
 4. Method according to either of claims 1 and 2, characterizedin that the reaction gas is a siloxane, preferably hexamethyldisiloxanein vapour form.
 5. Method according to any one of the preceding claims,characterized in that the reaction gas is injected at a nominal pressureof between 0.05 mbar (5 Pa) and 0.5 mbar (50 Pa).
 6. Method according toany one of the preceding claims, characterized in that the element orelements placed in the chamber comprise at least one element attachedthereto, such as a valve (15) or an electrical connector, in addition tothe tank casing.
 7. Element of a fuel system, especially a tank casing,comprising a substrate layer having an inner surface and an outersurface, characterized in that each of these surfaces is coated with atleast one material which is different from that of the substrate,arising from the decomposition of a chemical compound under the effectof plasma-generating microwave radiation.
 8. Element according to thepreceding claim, characterized in that the substrate layer comprises, oneach face, a layer of the same material or several layers of differentmaterials, the thickness of each layer being between 0.3 and 1 μm. 9.Element according to either of claims 7 and 8, characterized in that thecoating material comprises carbon with a polymer tendency and/or silica.10. Fuel tank made of high-density polyethylene comprising, on at leastone of its inner or outer faces, a coating of carbon and/or of silicawith a polymer tendency.